Recently, noises and vibration problems have become an object of public concern as an environmental pollution with development of means of transportation and increase in residential areas which are located near factories and the like. Further, in a workshop, there is a tendency to limit noises and vibration to improve working atmosphere. To cope with these tendencies, it is requested to impart vibration damping property to a metallic material which is a source of noises and vibration, that is, to impart a function to a noise generating member itself so that the member can absorb its own vibrational energy and convert it into heat energy to attenuate frequency of vibration or vibrational amplitude, thereby noise is decreased. Further, it is requested to improve such a function.
Based on these requests, as one of vibration damping materials having desired property, there has been proposed a vibration damping material having a composite laminate structure wherein a middle layer having viscoelasticity is sandwiched by metal layers. This type of a composite vibration damping material has been studied and employed as oil pans of automobiles, engine covers, chutes of hoppers, stopper of conveying apparatus, domestic electric equipments, vibration reducing members of other metal processing machines, structural members of precision machines in which prevention of vibration is desirable and the like.
In general, the vibration damping property of such a composite vibration damping material depends upon the property of a viscoelasticity layer which constitutes the middle layer thereof. When the vibration damping property is expressed as a loss factor (which is a measure of conversion of an external vibrational energy into a heat energy by internal friction, and is corresponding to a value relating to mechanical hysteresis loss due to vibration), the property shows a peak at a certain temperature. It has been known that it is most effective to use a vibration damping material at about this temperature showing the peak property.
Hitherto, as a viscoelastic composition which constitutes the middle layer of such a composite vibration damping material, there have been known a simple polyester (Japanese Patent Kokai No. 50-143880) or a polyester to which a plasticizer is added (Japanese Patent Kokai No. 51-93770); a simple polyurethane foam (Japanese Patent Kokai No. 51-91981); a simple polyamide (Japanese Patent Kokai No. 56-159160); a simple ethylene-polyvinyl acetate copolymer (Japanese Patent Kokai No. 57-34949); a composition of a polyvinyl butyral or a polyvinyl butyral and a polyvinyl acetate to which a plasticizer and a tackifier are added (Japanese Patent Kokoku No. 55-27975); a copolymer of a isocyanate prepolymer and a vinyl monomer (Japanese Patent Kokoku No. 52-26554); copolymers disclosed in Japanese Patent Kokoku Nos. 39-12451 and 45-34703(see counterpart U.S. Pat. No. 3,640,833), and Japanese Patent Kokai No. 62-74645; and the like.
Although, first of all, it is required that a composite vibration damping material should have a high value of the above loss factor as well as a high adhesive strength between a viscoelastic middle layer and a metal layer, the composite vibration damping material made of the above known viscoelastic composition has problems in any of these properties and is unsatisfactory. Particularly, in order to impart high vibration damping properties at about room temperature, it is necessary to set a glass transition temperature of a viscoelastic composition at below room temperature. However, when a glass transition temperature of a conventional resin is lowered, adhesion strength is also remarkably lowered. Therefore, it can not be used in a field requiring high adhesion.
Further, in a conventional vibration damping material, a temperature range in which vibration damping property is shown is very narrow and it is a problem in practice. For example, with the polyester or the mixture of the polyester and the plasticizer disclosed in the above Japanese Patent Kokai Nos. 50-143880 and 51-93770, its temperature range in which vibration damping property is shown is very narrow and, when it shows high vibration damping property at about room temperature, its adhesion strength is low. Both vibration damping property and adhesion strength of the polyester elastomer disclosed in the above Japanese Patent Kokai No. 62-74645 are insufficient. Particularly, when the polymer contains poly(alkylene oxide)glycol, heat resistance is also deteriorated, which results in limitation to usage. Further, by heating at baking finish, sometimes, adhesive power of a middle layer is lowered due to heat deterioration and peeling is caused.
Furthermore, in a conventional composite vibration damping material using a steel plate, durability is also insufficient. For example, a surface of the steel plate faced to the middle layer is rusted and adhesive force is lowered with time under various environmental conditions. Although it is effective to cure a middle layer to improve durability, vibration damping property is deteriorated by cure according to a conventional manner.
Thus, there has not been obtained a vibration damping material having both satisfactory vibration damping property and adhesion as well as good durability.
In the present assignees' U.S. Patent Application Ser. No. 085,718, there is disclosed a viscoelastic resin useful for a vibration damping material comprising a specific polyester diol, an aliphatic polyester diol, a diisocyante compound and a chain extender. This viscoelastic resin shows improved vibration damping property as well as improved adhesion when it is sandwiched between steel plates and improved press moldability when it is used as a middle layer of a composite vibration damping steel plate.
The present invention provides a viscoelastic resin useful for a middle layer of a composite vibration damping material having a wide effective temperature range of vibration damping property, particularly, having high vibration damping property even at about room temperature, and having good durability as well as maintaining high adhesive power even after baking finish.
This object as well as other objects and advantages of the present invention will become apparent to those skilled in the art from the following description.